1. Field of the Invention
The present invention relates to wireless communication techniques, and more particularly, to a wireless data transmitting and receiving apparatus and method using an UltraWide Band (hereinafter referred to as ‘UWB’).
2. Description of the Related Art
A wireless data transmission technique, which is adopted by cellular mobile communications, satellite communications, and television broadcast, transmits data by changing the waveform of a base frequency called a radio-frequency (RF) carrier and including the data into the RF carrier. A UWB technique is a data transmission method based on representation of data with 0's and 1's by repeatedly transmitting a pulse, which is an electric signal with a predetermined cycle and waveform, at intervals of time, which is shorter than 1 nanoseconds, without using the RF carrier.
In other words, the UWB technique transmits data using the pulse that acts as Morse code. When using very short pulses at predetermined intervals of time, e.g., per several hundred picoseconds, the pulses are modulated by a short length ±Dt of time before and after a predetermined time. When a pulse is modulated by a length −Dt of time, data is transmitted as 0's, and when a pulse is modulated by a length +Dt of time, data is transmitted as 1's. Transmission of coded pulses at precise points of time allows transmission of a lot of data and further increases the number of mobile users, theoretically without any restrictions.
FIG. 1 is a graph illustrating frequency spectrums of signals used in a narrow-band communications system, a wide-band CDMA system, and a UWB communications system. FIG. 1 illustrates the frequency spectrums of three different communications systems of the same output power. The three different communications systems include a narrowband communications system, a broadband Code Division Multiple Access (CDMA) system, and a UWB communications system. Referring to FIG. 1, compared to the narrowband communications system and the broadband CDMA system, the UWB communications system has lower density of spectrum power at a wide frequency band and thus can share frequencies with the existing wireless communications systems without interfering with the existing wireless communications systems.
The UWB communications system uses pulses with narrow widths of several nanoseconds or picoseconds. Accordingly, the system has very low density of spectrum power at an ultra wide frequency band, provides a high-security, a high data transmission rate, and undergoes less problems due to multi-path transmission. In particular, unlike the conventional wireless system, the UWB communications system is capable of conducting communications at a base band frequency without, carriers. Therefore, the structure of the UWB communications system is simple and makes it possible to manufacture a transceiver included in this system at a low cost.
That is, the UWB communications system transmits signal energy by dispersing the spectrum of the signal energy at a frequency of several GHz bandwidths not to interfere with other communications systems, thereby enabling communications without interfering with narrowband signals and irrespective of frequencies. The spectrum of a frequency domain is closely related to the shape of a signal waveform of a temporal domain. A sine wave has a large energy value at a particular frequency band, but an impulse has a uniform energy distribution over a wide frequency band. Thus, a pulse with a width of several nanoseconds or picoseconds is repeatedly used in the UWB communications.
FIG. 2 illustrates a pulse signal with a uniform amplitude distribution and the spectrum of the pulse signal.
When using only one pulse, an ultra low density of spectrum power appears as wideband noise over an ultra wide frequency band. However, as apparent from FIG. 2, if communications are made using a pulse with a uniform interval, the uniform periodicity of pulse causes an energy spark (comb line) to occur in the spectrum in a frequency domain. The energy spark may interfere with narrowband signals used in other communications and should be removed or minimized. To prevent the uniform periodicity of a pulse sequence, there is a need to change the interval of time between adjacent pulses.
A pulse and a pulse sequence, which is obtained by modulating the pulse, are required to transmit data 0′ and 1′ using a pulse. In general, a pulse is modulated using On-Off Keying (OFK), Pulse Amplitude Modulation (PAM), and Pulse Position Modulation (PPM).
FIG. 3 illustrates diagrams explaining PPM for transmission of a UWB signal and the spectrum of a position-modulated pulse. More specifically, the left drawing in FIG. 3 illustrates a method of changing the position of a pulse using PPM so as to make the pulse representation of signals 0 and 1. That is, a signal arriving before a reference point of time is expressed as 0's and a signal arriving after the reference point of time is expressed as 1's. The right drawing of FIG. 3 illustrates the spectrum of the pulse, the position of which is modulated using PPM. Referring to FIG. 3, RF energy of the pulse is more uniformly distributed over all frequency bands than the energy of the pulse shown in FIG. 2. Therefore, the existing narrowband communications system is less affected by the RF energy of FIG. 3. However, since only a portion of a pulse can be modulated using PPM, the position-modulated pulse assumes a similar shape to the spectrum of the previously explained pulse sequence having uniform duration. In conclusion, it is possible to make the spectrum uniform to some degree, but it is difficult to prevent the occurrence of the energy spark.
FIG. 4 illustrates a pulse with random interval and the spectrum of the pulse. More specifically, the left drawing in FIG. 4 illustrates a waveform of a position-modulated pulse with random intervals and the right drawing illustrates the energy spectrum of the pulse with random intervals.
When a transmitter generates a pulse with random interval and transmits it to a receiver via an antenna, the receiver creates a template pulse sequence representation of 0's or 1's and compares it with the received pulse sequence, estimates and acquires information regarding the received pulse sequence with random intervals based on the result of the comparison, and compares the received pulse and the template pulse sequence and obtains data from the received pulse based on the acquired information.
Therefore, the transmitter includes a random number generator that generates a random number sequence and the receiver also includes a random number generator that generates the same random number sequence. However, it is required to synchronize the random number sequence output from the transmitter and the random number sequence output from the receiver in order to interpret the received data.
FIG. 5 illustrates pulse sequences with random intervals. The synchronization represents that a transmitter and a receiver related to data transmission are to match a random-interval pulse sequence of a random number sequence generated by the receiver with a random-interval pulse sequence of a random number sequence generated by the transmitter, so as to check whether these random-interval pulse sequences are the same or not. If the energy distribution of the pulse sequence from the transmitter matches that of the pulse sequence from the receiver by 95% or more, these pulse sequences are considered as being identical with each other and information regarding the random number sequence used is stored.
Since the width of a UWB pulse is very small, its pulse energy is too slight to be detected. Thus, several pulses are sent to transmit a piece of data, thereby obtaining a favorable processing gain. That is, even if some of the sent pulses are lost, it is possible to restore the original information with the remaining pulses. However, a high-precision timer is further required to modulate and demodulate the UWB pulse, and it is difficult to restore the original information when only a portion of the UWB pulse changes.